use IO::Select;

# fire off all the ssh requests
my %ssh_handles = ();
foreach my $host ( keys %hosts ) {
    open my $ssh, '-|', "ssh $host 'echo $host'"
        or die "Can't ssh to $host: $!";
    $ssh_handles{$host} = $ssh;
}

# prepare to wait for them to finish
my $select = IO::Select->new();
$select->add( $_ ) for values %ssh_handles;

# wait until they're all done
my @ready;
while ( scalar @ready < scalar keys %ssh_handles ) {
    @ready = $select->can_read();
}

# @ready should be the same as values %ssh_handles
foreach my $host ( keys %hosts ) {
    my $ssh = $ssh_handles{$host};
    my $ssh_result = do { local $/; <$ssh> };
    print "$host said $ssh_result\n";
    close $ssh or warn "Can't close $host ssh: $!";
}
[download]

With some added complexity, you can tune how many of these you run at once (instead of all of them), you can output results as they come back (instead of only when they're all done). If you want, you can use select instead of IO::Select, but I find the module easier.

Update: Fixed a typo. Thanks cmv!

Use something like Parallel::ForkManager or write the multiprocess code yourself (see perlipc).

#!/usr/bin/perl
use strict;
use warnings;

my @hosts = ('first.host.com', 'second.host.com', 'third.host.com');

# Get the host we were called with
my $host = shift;
if (defined $host) {
    # Get the data from the specified host
    sshx (gather data);
}
else {
    # Called with a blank command line, so fire off a call for each 
    # host in our list
    for $host (@hosts) {
        # Fire off the same program, but give it a host and tell it to
        # run in the background
        system("data_gather.pl $host &");
    }
}
[download]

The basic idea is to open several ssh pipes (via open, open2 or open3) and collect the results using IO::Select and - if required - sending them additional information via the created stream channels.

Hope it helps

Casiano